Studies on modification of some first wall materials using 3-6.8 MeV He ions

Because very few results are reported in the MeV region, we have started a systematic investigation concerning the fluence and energy dependence of blistering induced in some useful materials for the first wall (stainless steels, Ni, Cu, Mo). They have been irradiated with 3.0, 4.7 and 6.8 MeV He ions. The irradiation effects have been investigated by means of a TEMSCAN-200-CX electron microscope and two metallographic microscopes. Irradiation phenomena as sponge- and wave-like structures, submicronic cracks, multilayer flaking, micro-conglomerates, secondary blisters, microcraters were observed. An interesting effect is the amorphous phase in the Ti-modified austenitic steel 12KH18N10T, phase formatted during irradiation with 6.8 MeV He+ ions up to a dose of crater occurrence (7x1018 ions per cm2), the temperature of specimens (100 μm thick) being maintained during irradiation in the range 30-60 deg. C. The X-ray microanalysis shows the presence of the major alloy elements in the amorphous area, whereas in that embedding a microcrystallite it is to be noticed the Ni depletion and the presence of Si and Mo. A potential explanation of the energy dependence of average blisters diameter d and blister critical dose could be stressed-induced model, because, in our cases (medium energy He ions), d-tm, where t is the blister skin thickness. For our high energy data, an m=1.8 value is relatively convenient, except Mo, where m=1.5 is more suitable